Functionalism

First published Tue Aug 24, 2004

Functionalism in the philosophy of mind is the doctrine that what
makes something a mental state of a particular type does not depend on
its internal constitution, but rather on the way it functions, or the
role it plays, in the system of which it is a part. This doctrine is
rooted in Aristotle's conception of the soul, and has antecedents in
Hobbes's conception of the mind as a “calculating machine”,
but it has become fully articulated (and popularly endorsed) only in
the last third of the 20th century. Though the term ‘functionalism’ is
used to designate a variety of positions in a variety of other
disciplines, including psychology, sociology, economics, and
architecture, this entry focuses exclusively on functionalism as a
philosophical thesis about the nature of mental states.

The following sections will trace the intellectual antecedents of
contemporary functionalism, sketch the different types of functionalist
theories, and discuss the most serious objections to them.

Functionalism is the doctrine that what makes something a thought,
desire, pain (or any other type of mental state) depends not on its
internal constitution, but solely on its function, or the role it
plays, in the cognitive system of which it is a part. More precisely,
functionalist theories take the identity of a mental state to be
determined by its causal relations to sensory stimulations, other
mental states, and behavior.

For (an avowedly simplistic) example, a functionalist theory might
characterize pain as a state that tends to be caused by bodily
injury, to produce the belief that something is wrong with the body and
the desire to be out of that state, to produce anxiety, and, in the
absence of any stronger, conflicting desires, to cause wincing or
moaning. According to this theory, all and only creatures with internal
states that meet these conditions, or play these roles, are capable of
being in pain.

Suppose that, in humans, there is some distinctive kind of neural
activity (C-fiber stimulation, for example) that meets these
conditions. If so, then according to this functionalist theory, humans
can be in pain simply by undergoing C-fiber stimulation. But the theory
permits creatures with very different physical constitutions to have
mental states as well: if there are silicon-based states of
hypothetical Martians or inorganic states of hypothetical androids that
also meet these conditions, then these creatures, too, can be in pain.
As functionalists often put it, pain can be realized by
different types of physical states in different kinds of creatures, or
multiply realized. (See entry on Multiple Realization.)
Indeed, since descriptions that make explicit reference only to a
state's causal relations with stimulations, behavior, and one another
are what have come to be known as “topic-neutral” (Smart,
1959) — that is, as imposing no logical restrictions on the
nature of the items that satisfy the descriptions — then it's
also logically possible for non-physical states to play the
relevant roles, and thus realize mental states, in some systems as
well. So functionalism is compatible with the sort of dualism that
takes mental states to cause, and be caused by, physical states.

Still, though functionalism is officially neutral between
materialism and dualism, it has been particularly attractive to
materialists, since many materialists believe (or argue; see Lewis,
1966) that it is overwhelmingly likely that any states capable of
playing the roles in question will be physical states. If so, then
functionalism can stand as a materialistic alternative to the
Psycho-Physical Identity Thesis, the thesis that each type of mental
state is identical with a particular type of neural state.
This thesis, once considered the dominant materialistic theory of the
mind, entails that no creatures with brains unlike ours can share our
sensations, beliefs, and desires, no matter how similar their behavior
and internal organization may be to our own. This is a consequence that
many regard as implausible (but see Hill, 1991, for a spirited
defense). Thus functionalism, with its claim that mental states can be
multiply realized, is widely regarded as providing a more inclusive,
less “(species-) chauvinistic” (Block, 1980) — and
thus more plausible — theory that is (at least arguably)
compatible with materialism.

Within this broad characterization of functionalism, however, a
number of distinctions can be made. One of particular importance is the
distinction between theories in which the functional characterizations
of mental states purport to provide analyses of the meanings of our
mental state terms, and theories that permit functional
characterizations of mental states to appeal to information deriving
from scientific experimentation (or speculation). (See Shoemaker,
1984c, and Rey, 1997, for further discussion and more fine-grained
distinctions.) There are other important differences among
functionalist theories as well. These (sometimes orthogonal)
differences, and the motivations for them, can best be appreciated by
examining the origins of functionalism and tracing its evolution in
response both to explicit criticisms of the thesis and changing views
about the nature of psychological explanation.

Although functionalism attained its greatest prominence as a theory
of mental states in the last third of the 20th century, it has
antecedents in both modern and ancient philosophy, as well as in early
theories of computation and artificial intelligence.

The earliest view that can be considered an ancestor of
functionalism is Aristotle's theory of the soul (350 BCE). In contrast
to Plato's claim that the soul can exist apart from the body, Aristotle
argued (De Anima Bk. II, Ch. 1) that the (human) soul is the
form of a natural, organized human body — the set of
powers or capacities that enable it to express its “essential
whatness”, which for Aristotle is a matter of fulfilling the
function or purpose that defines it as the kind of thing it is. Just as
form of an axe is whatever enables it to cut, and the form of an eye is
whatever enables it to see, the (human) soul is to be identified with
whichever powers and capacities enable a natural, organized human body
to fulfill its defining function, which, according to Aristotle, is to
survive and flourish as a living, acting, perceiving, and reasoning
being. So, Aristotle argues, the soul is inseparable from the body, and
comprises whichever capacities are required for a body to live,
perceive, reason, and act.

A second, relatively early, ancestor of contemporary functionalism
is Hobbes's (1651) account of reasoning as a kind of computation that
proceeds by mechanistic principles comparable to the rules of
arithmetic. Reasoning, he argues, is “nothing but
reckoning, that is adding and subtracting, of the consequences
of general names agreed upon for the marking and
signifying of our thoughts.” (Leviathan, Ch. 5)
In addition, Hobbes suggests that reasoning — along with
imagining, sensing, and deliberating about action, all of which proceed
according to mechanistic principles — can be performed by systems
of various physical types. As he puts it in his Introduction to
Leviathan, where he likens a commonwealth to an individual
human, “why may we not say that all automata (engines that move
themselves by springs and wheels…) have an artificial life? For
what is the heart but a spring; and the nerves but so many strings, and
the joints but so many wheels…”. It was not until the
middle of the 20th century, however, that it became common to speculate
that thinking may be nothing more than rule-governed computation that
can be carried out by creatures of various physical types.

In a seminal paper (Turing, 1950), A.M. Turing proposed that the
question, “Can machines think?” can be replaced by the
question, “Is it theoretically possible for a finite state
digital computer, provided with a large but finite table of
instructions, or program, to provide responses to questions that would
fool an unknowing interrogator into thinking it is a human
being?” Now, in deference to its author, this question is most
often expressed as “Is it theoretically possible for a finite
state digital computer (appropriately programmed) to pass the Turing
Test?” (See Turing Test entry)

In arguing that this question is a legitimate replacement for the
original (and speculating that its answer is “yes”), Turing
identifies thoughts with states of a system defined solely by their
roles in producing further internal states and verbal outputs, a view
that has much in common with contemporary functionalist theories.
Indeed, Turing's work was explicitly invoked by many theorists during
the beginning stages of 20th century functionalism, and was the avowed
inspiration for a class of theories, the “machine state”
theories most firmly associated with Hilary Putnam (1960, 1967), that
had an important role in the early development of the doctrine.

Other important recent antecedents of functionalism are the
behaviorist theories that emerged in the early-to-mid twentieth
century. These include both the empirical psychological theories
associated primarily with Watson and Skinner, and the
“logical” or “analytical” behaviorism of
philosophers such as Malcolm (1968) and Ryle (1949) (and, arguably,
Wittgenstein, 1953). Though functionalism is significantly different
from behaviorism in that the latter attempts to explain behavior
without any reference whatsoever to mental states and processes, the
development of two important strains of functionalism,
“psychofunctionalism” and “analytical”
functionalism, can both be profitably viewed as attempts to rectify the
difficulties, respectively, of empirical and logical behaviorism, while
retaining certain important insights of those theories.

As an empirical psychological theory, behaviorism holds that the
behavior of humans (and other animals) can be explained by appealing
solely to behavioral dispositions, that is, to the lawlike tendencies
of organisms to behave in certain ways, given certain environmental
stimulations. Behavioral dispositions, unlike thoughts, feelings, and
other internal states that can be directly observed only by
introspection, are objectively observable and are indisputably part of
the natural world. Thus they seemed to be fit entities to figure
centrally in the emerging science of psychology. Also, behaviorist
theories promised to avoid a potential regress that appeared to
threaten psychological explanations invoking internal representations,
namely, that to specify how such representations produce the behaviors
in question, one must appeal to an internal intelligent agent (a
“homunculus”) who interprets the representations, and whose
skills themselves would have to be explained.

The promise of behaviorism lay in its conviction that there could be
a science of human behavior as objective and explanatory as other
“higher-level” sciences such as chemistry and biology.
Behaviorism indeed had some early successes, especially in the domain
of animal learning, and its principles are still used, at least for
heuristic purposes, in various areas of psychology. But as many
psychologists (and others, e.g. Chomsky 1959) have argued, the
successes of behaviorism seem to depend upon the experimenters'
implicit control of certain variables which, when made explicit,
involve ineliminable reference to organisms' other mental states. For
example, rats are typically placed into an experimental situation at a
certain fraction of their normal body weight — and thus can be
assumed to feelhunger and to want the food
rewards contingent upon behaving in certain ways. Similarly, it is
assumed that humans, in analogous experimental situations,
want to cooperate with the experimenters, and
understand and know how to follow the instructions. It seemed
to the critics of behaviorism, therefore, that theories that explicitly
appeal to an organism's beliefs, desires, and other mental states, as
well as to stimulations and behavior, would provide a fuller and more
accurate account of why organisms behave as they do. They could do so,
moreover, without compromising the objectivity of psychology as long as
the mental states to which these theories appeal are introduced as
states that together play a role in the production of
behavior, rather than states identifiable solely by introspection. Thus
work was begun on a range of “cognitive” psychological
theories which reflected these presumptions, and an important strain of
contemporary functionalism, “psycho-functionalism” (Fodor,
1968, Block and Fodor, 1972) can be seen a philosophical endorsement of
these new cognitive theories of mind.

Logical behaviorism, in contrast to behaviorism as a psychological
theory, is a thesis about the meanings of our mental state terms or
concepts. According to logical behaviorism, all statements about mental
states and processes are equivalent in meaning to statements about
behavioral dispositions. So, for (again, an overly simplified) example,
“Henry has a toothache” would be equivalent in meaning to a
statement such as “Henry is disposed (all things being equal) to
cry out or moan and to rub his jaw”. And “Amelia is
thirsty” would be equivalent to a statement such as “If
Amelia is offered some water, she will be disposed (all things being
equal) to drink it.” These candidate translations, like all
behavioristic statements, eschew reference to any internal states of
the organism, and thus do not threaten to denote, or otherwise induce
commitment to, properties or processes (directly) observable only by
introspection. In addition, logical behaviorists argued that if
statements about mental states were equivalent in meaning to statements
about behavioral dispositions, there could be an unproblematic account
of how mental state terms could be applied both to oneself and others,
and how they could be taught and learned.

However, as many philosophers have pointed out (Chisholm, 1957;
Geach, 1957), logical behaviorism provides an implausible account of
the meanings of our mental state terms, since, intuitively, a subject
can have the mental states in question without the relevant behavioral
dispositions — and vice versa. For example, Gene may believe that
it's going to rain even if he's not disposed to wear a raincoat and
take an umbrella when leaving the house (or to perform any other
cluster of rain-avoiding behaviors), if Gene doesn't mind, or actively
enjoys, getting wet. And subjects with the requisite motivation can
suppress their tendencies to pain behavior even in the presence of
excruciating pain, while skilled actors can perfect the lawlike
disposition to produce pain behavior under certain conditions, even if
they don't actually feel pain. (Putnam, 1965) The problem, these
philosophers argued, is that no mental state, by itself, can be
plausibly assumed to give rise to any particular behavior unless one
also assumes that the subject possesses additional mental states of
various types. And so, it seemed, it was not in fact possible to give
meaning-preserving translations of statements invoking pains, beliefs,
and desires in purely behavioristic terms. Nonetheless, the idea that
the meanings of mental state terms and concepts show an essential tie
between mental states and their typical behavioral expressions is
retained, and elaborated, in contemporary “analytic”
functionalist theories.

As suggested earlier, it is helpful to think of functionalist
theories as belonging to one of three major strains —
“machine functionalism”, “psychofunctionalism”
and “analytic functionalism” — and to see them as
emerging, respectively, from early AI theories, empirical behaviorism,
and logical behaviorism. It's important to recognize, however, that
there is at least some overlap in the bloodlines of these different
strains of functionalism, and also that there are functionalist
theories, both earlier and more recent, that fall somewhere in between.
For example, Wilfrid Sellars's (1956) account of mental states as
“theoretical entities” is widely regarded as an important
early version of functionalism, but it takes the proper
characterization of thoughts and experiences to depend partially on
their role in providing a scientific explanation of behavior, and
partly on what he calls the “logic”, or the a priori
interrelations, of the relevant concepts. Still, it is instructive to
give separate treatment to the three major strains of the doctrine, as
long as these caveats are kept in mind.

The early functionalist theories of Putnam (1960, 1967) can be seen
as a response to the difficulties facing behaviorism as a scientific
psychological theory, and as an endorsement of the (new) computational
theories of mind which were becoming increasingly significant rivals to
it. According to Putnam's machine state functionalism, any
creature with a mind can be regarded as a Turing machine (an idealized
finite state digital computer), whose operation can be fully specified
by a set of instructions (a “machine table” or program)
each having the form:

If the machine is in state Si, and
receives input Ij, it will go into state
Sk and produce output Ol (for a
finite number of states, inputs and outputs).

A machine table of this sort describes the operation of a
deterministic automaton, but most machine state functionalists
(e.g. Putnam 1967) take the proper model for the mind to be that of a
probabilistic automaton: one in which the program specifies,
for each state and set of inputs, the probability with which
the machine will enter some subsequent state and produce some
particular output.

On either model, however, the mental states of a creature are to be
identified with such “machine table states”
(S1,…,Sn). These states
are not mere behavioral dispositions, since they are specified in terms
of their relations not only to inputs and outputs, but also to the
state of the machine at the time. For example, if believing it will
rain is regarded as a machine state, it will not be regarded as a
disposition to take one's umbrella after looking at the weather report,
but rather as a disposition to take one's umbrella if one looks at the
weather report and is in the state of wanting to stay dry. So
machine state functionalism can avoid what many have thought to be a
fatal difficulty for behaviorism. In addition, machines of this sort
provide at least a simple model of how internal states whose effects on
output occur by means of mechanical processes can be viewed as
representations (though the question of what,
exactly, they represent has been an ongoing topic of discussion (see
sections 4.4-5). Finally, machine table states are not tied to any
particular physical (or other) realization; the same program, after
all, can be run on different sorts of computer hardware.

It's easy to see, therefore, why Turing machines provided a fruitful
model for early functionalist theories. But because machine table
states are total states of a system, the early functionalist equation
of mental states with machine table states faded in importance as a
model for the functional characterization of the complex of distinct
internal states that can be simultaneously realized in a human (or
other) subject (Block and Fodor, 1972; Putnam, 1973). Nonetheless, the
idea that internal states can be fully described in terms of their
relations to input, output, and one another, and can figure in
lawlike descriptions, and predictions, of a system's output, was a rich
and important idea that is retained by contemporary functionalist
theories.

A second strain of functionalism, psycho-functionalism, derives
primarily from reflection upon the goals and methodology of
“cognitive” psychological theories. In contrast to the
behaviorists' insistence that the laws of psychology appeal only to
behavioral dispositions, cognitive psychologists argue that the best
empirical theories of behavior take it to be the result of a complex of
mental states and processes, introduced and individuated in terms of
the roles they play in producing the behavior to be explained. For
example (Fodor's, in his 1968, Ch. 3), a psychologist may begin to
construct a theory of memory by postulating the existence of
“memory trace” decay, a process whose occurrence or absence
is responsible for effects such as memory loss and retention, and which
is affected by stress or emotion in certain distinctive ways.

On a theory of this sort, what makes some neural process an instance
of memory trace decay is a matter of how it functions, or the role it
plays, in a cognitive system; its neural or chemical properties are
relevant only insofar as they enable that process to do what trace
decay is hypothesized to do. And similarly for all mental states and
processes invoked by cognitive psychological theories. Cognitive
psychology, that is, is intended by its proponents to be a
“higher-level” science like biology: just as, in biology,
physically disparate entities can all be hearts as long as they
function to circulate blood in a living organism, and physically
disparate entities can all be eyes as long as they enable an organism
to see, disparate physical structures or processes can be instances of
memory trace decay — or more familiar phenomena such as thoughts,
sensations, and desires — as long as they play the roles
described by the relevant cognitive theory.

Psycho-functionalism, therefore, can be seen as straightforwardly
adopting the methodology of cognitive psychology in its
characterization of mental states and processes as entities defined by
their role in a cognitive psychological theory. All versions of
functionalism, however, can be regarded as characterizing mental states
in terms of their roles in some psychological theory or other.
(A more formal account of this will be given in Section 4.1 below.)
What is distinctive about psycho-functionalism is its claim that mental
states and processes are just those entities, with just those
properties, postulated by the best scientific explanation of
human behavior. This means, first, that the form of the theory can
diverge from the “machine table” specifications of machine
state functionalism. It also means that the information used in the
functional characterization of mental states and processes needn't be
restricted to what is considered common knowledge or common sense, but
can include information available only by careful laboratory
observation and experimentation. For example, a psychofunctional theory
might be able to distinguish phenomena such as depression from sadness
or listlessness even though the distinctive causes and effects of these
syndromes are difficult to untangle solely by consulting intuitions or
appealing to common sense. And psychofunctional theories will not
include characterizations of mental states for which there is no
scientific evidence, such as buyer's regret or hysteria, even if the
existence and efficacy of such states is something that common sense
affirms.

This may seem to be an unmitigated advantage, since
psycho-functional theories can avail themselves of all the tools of
inquiry available to scientific psychology, and will presumably make
all, and only, the distinctions that are scientifically sound. This
methodology, however, leaves psycho-functionalism open to the charge
that it, like the Psycho-Physical Identity Thesis, may be overly
“chauvinistic” (Block, 1980), since creatures whose
internal states share the rough, but not fine-grained, causal patterns
of ours wouldn't count as sharing our mental states. Many
psycho-functionalists may not regard this as an unhappy consequence,
and argue that it's appropriate to treat only those who are
psychologically similar as having the same mental states. But there is
a more serious worry about the thesis, namely, that if the laws of the
best empirical psychological theories diverge from even the broad
contours of our “folk psychology” — that is, our
common sense beliefs about the causal roles of our thoughts,
sensations, and perceptions — it will be hard to take
psycho-functional theories as providing an account of our mental states
(Loar, 1981). Many theorists, however (Horgan and Woodward 1985), argue
that it's likely that future psychological theories will be
recognizably close to “folk psychology”, though this
question has been the subject of debate (Churchland, 1981).

But there is another important strain of functionalism,
“analytic” functionalism, that takes there to be reason to
restrict the defining theory not just to generalizations sufficiently
close to those that “the folk” take to hold between mental
states, environmental stimulations, and behavior, but rather to a
priori information about these relations. (See Smart (1959),
Armstrong (1968), Shoemaker (1984a,b,c), Lewis (1972).) This is
because, for analytic functionalists, there are equally important goals
that require strictly a priori characterizations of mental states.

Like the logical behaviorism from which it emerged, the goal of
analytic functionalism is to provide “topic-neutral”
translations, or analyses, of our ordinary mental state terms or
concepts. Analytic functionalism, of course, has richer resources than
logical behaviorism for such translations, since it permits reference
to the causal relations that a mental state has to stimulations,
behavior, and other mental states. Thus the statement
“Blanca wants some coffee” need not be rendered, as logical
behaviorism requires, in terms such as “Blanca is disposed to
order coffee when it is offered”, but rather as “Blanca is
disposed to order coffee when it is offered, if she has no
stronger desire to avoid coffee”. But this requires any
functional “theory” acceptable to analytic functionalists
to include only generalizations about mental states, their
environmental causes, and their joint effects on behavior that are so
widely known and “platitudinous” as to count as
analyzing our ordinary concepts of the mental states in
question.

A good way to see why analytic functionalists insist that functional
characterizations provide meaning analyses is to revisit a debate that
occurred in the early days of the Psycho-Physical Identity Theory, the
thesis that each type of mental state can be identified with some type
of brain state or neural activity. For example, early identity
theorists (Smart, 1962; Place, 1956) argued that it makes perfect sense
(and may well be true) to identify pain with C-fiber
stimulation. The terms ‘pain’ and ‘C-fiber stimulation’, they
acknowledged, do not have the same meaning, but nonetheless
they can denote the same state; the fact that an identity statement is
not a priori, they argued, does not mean that it is not true. And just
because I need not consult some sort of brain scanner when reporting
that I'm in pain doesn't mean that the pain I report is not a neural
state that a brain scanner could (in principle) detect.

An important — and enduring — objection to this
argument, however, was raised early on by Max Black (reported in Smart,
1962). Black argued, following Frege (1892), that the only way that
terms with different meanings can denote the same state is to express
different properties, or “modes of presentation” of that
state. But this implies, he argued, that if terms like ‘pain’,
‘thought’ and ‘desire’ are not equivalent in meaning to any
physicalistic descriptions, they can denote physical states only by
expressing irreducibly mentalproperties of them.
Thus, even if ‘pain’ and ‘C-fiber stimulation’ pick out a single type
of neural state, this state must have two types of properties,
physical and mental, by means of which the identification can be made.
This argument has come to be known as the “Distinct Property
Argument”, and is taken by its proponents to undermine a
thorough-going materialistic theory of the mind. (See White, 1986,
2002, for more recent versions of this argument.)

The appeal of meaning-preserving functional characterizations,
therefore, is that in providing topic-neutral equivalents of our mental
state terms and concepts, they blunt the anti-materialistic force of
the Distinct Property Argument. True enough, analytic functionalists
can acknowledge, terms like ‘pain’, ‘thought’, and ‘desire’ are not
equivalent to any descriptions expressed in the language of physics,
chemistry, or neurophysiology. But if there are functional
descriptions that preserve the meanings of these terms, then a
creature's mental states can be identified simply by determining
whether that creature has internal states and processes that can play
the relevant functional roles. And since the capacity to play these
roles is merely a matter of having certain causal relations to
stimulations, behavior, and one another, the possession of these
properties is compatible with a materialistic theory of the mind.

A major question, of course, is whether a theory that limits itself
to a priori information about the causal relations between
stimulations, mental states and behavior can make the right
distinctions among mental states. The relative strengths and weaknesses
of analytic and psycho-functionalism will be discussed further in later
sections. First, however, there is another important distinction
between kinds of functional theory — one that crosscuts the
distinctions described so far — that is important to note. This
is the distinction between functional specifications and Functional
State Identity Theories.

To see the difference between these types of theories, consider the
(avowedly simplistic) example of a functional theory of pain introduced
in the first section:

Pain is the state that tends to be caused by
bodily injury, to produce the belief that something is wrong with the
body and the desire to be out of that state, to produce anxiety, and,
in the absence of any stronger, conflicting desires, to cause wincing
or moaning.

As noted earlier, if in humans this functional role is played by
C-fiber stimulation, then, according to this functionalist theory,
humans can be in pain simply by undergoing C-fiber stimulation. But
there is a further question to be answered, namely, what is the
property of pain itself? Is it the (second-order relational) property
of being in some state or other that plays the “pain role”
in the theory, or the C-fiber stimulation that actually plays this
role?

A Functional State Identity Theory (FSIT) would identify pain (or,
more naturally, the property of having a pain or being in
pain) with the second-order relational property. Other theorists,
however, take a functional theory merely to provide definite
descriptions of whichever first-order physical (or other)
properties satisfy the functional characterizations and for those
properties themselves to be the pains, beliefs, and desires. On this
view, if the property that occupies the functional role of pain in
human beings is C-fiber stimulation, then pain (or at least
pain-in-humans) would be C-fiber stimulation (rather than the
property of having some first-order state that plays the relevant
role). Views of this sort have come to be known as “functional
specification” theories, since mental states, though identified
with particular physical properties, are specified in terms of their
functional roles. Such views could equally well be considered to be
versions of the psycho-physical identity thesis, but they are most
often discussed along with FSIT's, since both consider it essential to
be able to characterize mental states in functional terms. (This is not
to suggest that there is a difference in kind between higher-order
“role” properties and the lower-order
“realizations” of those roles, since it may be that,
relative to even lower-level descriptions, those realizations can be
characterized as functional states themselves (Lycan (1987)).

It's clear why psycho-physical identity theorists worried about the
Distinct Property argument (see section 3.3) would adopt a functional
specification theory, in hopes that functional specification could
provide topic-neutral translations of mental state terms and concepts.
Indeed, the earliest versions of analytic functionalism (Smart, 1959,
Armstrong, 1968) were functional specification theories rather than
FSIT's. But some psychofunctionalists also prefer functional
specification theories to FSIT's, since they appear to offer a more
straightforward account of the causal relations between stimulations,
mental states, and behavior. Taking mental states to be the first order
state-types which, in each species, satisfy the functional definitions
makes it possible to say, literally, that pain (given the
presence, or absence, of certain other mental states) causes
wincing, whereas on the view that pain is a second-order property
partially defined by its tendency to produce wincing, one can say only
that wincing is a manifestation of pain.

On the other hand, taking mental states to be the second-order
properties expressed by the functional definitions permits us,
literally, to count as having the same mental states as creatures who
realize the functional definitions in different ways, and on this view
mental state terms can be rigid designators (Kripke, 1972), denoting
the same items — those second-order properties — in all
possible worlds. Functional specification theories and FSITs appear,
that is, to have different strengths and weaknesses:
straightforwardness of causal explanation versus universality of
application (Block, 1980).

Evaluating the significance of these differences, however, is a
complicated matter. Even though, on the functional specification view,
one can't state that all individuals in a state that plays the
functional role of pain are literally in the same mental state, one can
attribute to all of them the closely related second-order property
(call it “being in a state of pain”) possessed by all and
only individuals with first-order state types that satisfy the
functional definition. This may be generalization enough. And the
straightforward ‘pain causes wincing’ that is possible on the
functional specification view can be replaced, on the FSIT, by a
locution such as ‘wincing occurs because of pain’, which may
provide sufficient explanation of the relation between mental state and
behavior. (See Mumford (1998) for a general discussion of the
conditions under which dispositions can be said to cause
manifestations, or be caused by the environmental stimuli in terms of
which they are defined.)

This issue leads to some difficult questions about what exactly is
required for a mental property to have causal efficacy, since there are
problems in accounting for “mental causation” whether
mental states are taken to be identical to, realized by, or
supervenient on, physical properties. But though this topic is briefly
revisited in Section 6, a full discussion of these questions is beyond
the scope of this entry. (See Mental Causation entry; also Kim,
2002.)

So far, the discussion of how to provide functional
characterizations of individual mental states has been vague, and the
examples avowedly simplistic. Is it possible to do better, and, if so,
which version of functionalism is likely to have the greatest success?
These questions will be the focus of this section, and separate
treatment will be given to intentional states, such as
thoughts, beliefs, and desires, which purport to represent the world in
various ways, and experiential states, such as perceptions and
bodily sensations, which have a distinctive qualitative character or
“feel” (though these groups may not be mutually
exclusive).

First, however, it is important to get more precise about how
exactly functional definition is supposed to work. This can be done by
focusing on a general method for providing functional definitions
introduced by David Lewis (1972; building on an idea of Frank
Ramsey's), and which has become standard practice for functionalists of
all varieties.

The key feature of this now-canonical method is to treat mental
states and processes as being implicitly defined by the Ramsey
sentence of one or another psychological theory — common
sense, scientific, or something in between. (Analogous steps, of
course, can be taken to produce the Ramsey-sentence of any
theory, psychological or otherwise). For (a still simplistic) example,
consider the sort of generalizations about pain introduced before: pain
tends to be caused by bodily injury; pain tends to produce the belief
that something is wrong with the body and the desire to be out of that
state; pain tends to produce anxiety; pain tends to produce wincing or
moaning.

To construct the Ramsey-sentence of this “theory”, the
first step is to conjoin these generalizations, then replace all names
of different types of mental states with different variables, and then
existentially quantify those variables, as follows:

Such a statement is free of any mental state terms. It includes
quantifiers that range over mental states, terms that denote
stimulations and behavior, and terms that attribute various causal
relations to them. It can thus be regarded as providing implicit
definitions of the mental state terms of the theory. An individual will
have those mental states just in case it possesses a family of
first-order states that interact in the ways specified by the theory.
(Though functionalists of course acknowledge that the first-order
states that satisfy the functional definitions may vary from species to
species — or even from individual to individual — they
specify that, for each individual, the functional definitions be
uniquely satisfied.)

A helpful way to think of the Ramsey sentence of a psychological
theory is to regard it as defining a system's mental states “all
at once” as states that interact with stimulations in various
ways to produce behavior (Lewis, 1972; also see Field (1980) for a more
technical elaboration of Lewis's method, and an account of some crucial
differences between this kind of characterization and the one Lewis
initially proposed.) This makes clear that, in the classic formulations
of functional theories, a given mental state is intended to be
characterized in terms of its relations to stimulations, behavior, and
all the other states invoked by the theory in question. But
herein lies a difficulty that presents a challenge to any version of
functionalism.

The difficulty is that the characterization of mental states in (all
the versions of) functionalism so far presented is holistic.
According to functionalism, mental states are to be characterized in
terms of their roles in a psychological theory, but psychological
theories, whether common sense or scientific, incorporate information
about a large number and variety of mental states. Thus if pain is
interdefined with certain highly articulated beliefs and desires, then
animals who don't have internal states that play the roles of our
articulated beliefs and desires can't share our pains, and humans
without the capacity to feel pain can't share certain (or perhaps any)
of our beliefs and desires. In addition, differences in the ways people
reason, the ways their beliefs are fixed, or the ways their desires
affect their beliefs — due either to cultural or individual
idiosyncracies — might make it impossible for them to share the
same mental states. These are regarded as serious worries for all
versions of functionalism (see Stich, 1983, Putnam, 1988).

Some functionalists, however (e.g. Shoemaker, 1984c), have suggested
that if a creature has states that approximately realize our
functional theories, or realize some more specific defining
subset of the theory particularly relevant to the specification of
those states, then they can qualify as being mental states of the same
types as our own. The problem, of course, is to specify more precisely
what it is to be an approximate realization of a theory, or what
exactly a “defining” subset of a theory is intended to
include, and these are not easy questions. (They have particular bite,
moreover, for analytic functionalist theories, since
specifying what belongs inside and outside the “defining”
subset of a functional characterization raises the question of what are
the conceptually essential, and what the merely collateral, features of
a mental state, and thus raise serious questions about the feasibility
of (something like) an analytic-synthetic distinction. (Quine, 1953,
Rey, 1997)).

In addition to these general worries arising from the holism of
Ramsey-style functional characterizations, there are particular
questions that arise for the projects of giving functional
characterizations of experiential and intentional
states. These questions will be addressed in the following three
sections.

The key strategy in the most successful treatments of perceptual
experiences and bodily sensations (Shoemaker, 1984a Clark, 1993;
adumbrated in Sellars, 1956) is to individuate experiences of various
general types (color experiences, experiences of sounds, feelings of
temperature) in part by appeal to their positions in the “quality
spaces” associated with the relevant sense modalities —
that is, the (perhaps multidimensional) matrices determined by
judgments about the relative similarities and differences among the
experiences in question. So, for example, the experience of a very
reddish-orange could be (partially) characterized as the state produced
by the viewing of a color swatch within some particular range, which
tends to produce the judgment or belief that the state just experienced
is more similar to the experience of red than of orange. (Analogous
characterizations, of course, will have to be given of these other
color experiences.) The judgments or beliefs in question will
themselves be (partially) characterized in terms of their tendencies to
produce sorting or categorization behavior of certain specified
kinds.

This strategy may seem fatal to analytic functionalism, which
restricts itself to the use of a a priori information to distinguish
among mental states, since it's not clear that the information needed
to distinguish among experiences such as color perceptions will result
from conceptual analysis of our mental state terms or concepts.
However, this problem may not be as dire as it seems. For example, if
sensations and perceptual experiences are characterized in terms of
their places in a “quality space” determined by a person's
pre-theoretical judgments of similarity and dissimilarity (and perhaps
also in terms of their tendencies to produce various emotional
effects), then these characterizations may qualify as a
priori, even though they would have to be elicited by a kind of
“Socratic questioning”.

There are limits to this strategy, however (see Section 5.1 on the
“inverted spectrum” problem), which seem to leave two
options for analytic functionalists: fight — that is, deny that
it's coherent to suppose that there are the distinctions that the
critics suggest, or switch — that is, embrace another version of
functionalism in which the characterizations of mental states, though
not conceptual truths, can provide information rich enough to
individuate the states in question. To switch, however, would be to
give up the benefits (if any; again see Section 5.1) of a theory that
offers meaning-preserving translations of our mental state terms.

There has been significant skepticism, however, about whether any
functionalist theory — analytic or scientific — can capture
what seem to be the intrinsic characters of experiential states such as
color perceptions, pains, and other bodily sensations; these questions
will be addressed in section 5.1 below.

On the other hand, intentional states such as beliefs, thoughts, and
desires (sometimes called “propositional attitudes”) are
often taken to be easier to specify in functional terms (but not
always: see Searle, 1992, G. Strawson, 1994, who suggest that
intentional states have qualitative character as well). It's not hard
to see how to begin: beliefs are (among other things) states produced
in certain ways by sense-perception or inference from other beliefs,
and which tend to interact with certain desires to produce behavior;
desires are states with certain causal or counterfactual relations to
the system's goals and needs, and which tend to interact with certain
beliefs to produce behavior. But more must be said about what makes a
state a particular belief or desire, for example, the belief — or
desire — that it will snow tomorrow. Most functional theories
describe such states as different relations (or
“attitudes”) toward the same state of affairs or
proposition (and to describe the belief that it will snow tomorrow and
the belief that it will rain tomorrow as the same attitude toward
different propositions). This permits differences and similarities in
the contents of intentional states to be construed as differences and
similarities in the propositions to which these states are related. But
what makes a mental state a relation to, or attitude toward, some
proposition P? And can these relations be captured solely by appeal to
the functional roles of the states in question?

The development of conceptual role semantics may seem to provide an
answer to these questions: what it is for Julian to believe that P is
for Julian to be in a state that has causal and counterfactual
relations to other beliefs and desires that mirror certain inferential,
evidential, and practical (action-directed) relations among
propositions with those formal structures (Field, 1980; Loar, 1981;
Block, 1986). This proposal raises a number of important questions. One
is whether states capable of entering into such interrelations can
(must?) be construed as comprising, or including elements of, a
“language of thought” (Fodor, 1975; Harman, 1973; Field,
1980; Loar, 1981). Another is whether idiosyncracies in the inferential
or practical proclivities of different individuals make for differences
in (or incommensurabilities between) their intentional states. (This
question springs from a more general worry about the holism of
functional specification, which was discussed more generally in Section
4.2.)

Yet another challenge for functionalism are the widespread
intuitions that support “externalism”, the thesis that what
mental states represent, or are about, cannot be characterized without
appeal to certain features of the environments in which those
individuals are embedded. Thus, if one individual's environment differs
from another's, they may count as having different intentional states,
even though they reason in the same ways, and have exactly the same
“take” on those environments from their own points of
view.

The “Twin Earth” scenarios introduced by Putnam (1975)
are often invoked to support an externalist individuation of beliefs
about natural kinds such as water, gold, or tigers. Twin Earth, as
Putnam presents it, is a (hypothetical) planet on which things look,
taste, smell and feel exactly the way they do on Earth, but which have
different underlying microscopic structures; for example, the stuff
that fills the streams and comes out of the faucets, though it looks
and tastes like water, has molecular structure XYZ rather than
H2O. Many theorists find it intuitive to think that we
thereby mean something different by our term ‘water’ than our
Twin Earth counterparts mean by theirs, and thus that the beliefs we
describe as beliefs about water are different from those that our Twin
Earth counterparts would describe in the same way. Similar conclusions,
they contend, can be drawn for all cases of belief (and other
intentional states) regarding natural kinds.

The same problem, moreover, appears to arise for other sorts of
belief as well. Tyler Burge (1979) presents cases in which it seems
intuitive that a person, Oscar, and his functionally equivalent
counterpart have different beliefs about various syndromes (such as
arthritis) and artifacts (such as sofas) because the usage of these
terms by their linguistic communities differ. For example, in Oscar's
community, the term ‘arthritis’ is used as we use it,
whereas in his counterpart's community ‘arthritis’ denotes
inflammation of the joints and also various maladies of the thigh.
Burge's contention is that even if Oscar and his counterpart both
complain about the ‘arthritis’ in their thighs and make
exactly the same inferences involving ‘arthritis’, they
mean different things by their terms and must be regarded as having
different beliefs. If these cases are convincing, then there are
differences among types of intentional states that can only be captured
by characterizations of these states that make reference to the
practices of an individual's linguistic community. These, along with
the Twin Earth cases, suggest that if functionalist theories cannot
make reference to an individual's environment, then capturing the
representational content of (at least some) intentional states is
beyond the scope of functionalism. (See Section 4.5 for further
discussion, and Searle, 1980, for related arguments against
“computational” theories of intentional states.)

On the other hand, the externalist individuation of intentional
states may fail to capture some important psychological commonalities
between ourselves and our counterparts that are relevant to the
explanation of behavior. If my Twin Earth counterpart and I have both
come in from a long hike, declare that we're thirsty, say “I want
some water” and head to the kitchen, it seems that our behavior
can be explained by citing a common desire and belief. Some theorists,
therefore, have suggested that functional theories should attempt
merely to capture what has been called the “narrow content”
of beliefs and desires — that is, whichever representational
features individuals share with their various Twin Earth counterparts.
There is no consensus, however, about just how functionalist theories
should treat these “narrow” representational features
(Block, 1986; Loar, 1987), and some philosophers have expressed
skepticism about whether such features should be construed as
representations at all (Fodor, 1994; also see entry on Narrow Content).
Even if a generally acceptable account of narrow representational
content can be developed, however, if the intuitions inspired by
“Twin Earth” scenarios remain stable, then one must
conclude that the full representational content of intentional states
(and qualitative states, if they too have representational content)
cannot be captured by “narrow” functional characterizations
alone.

Considerations about whether certain sorts of beliefs are to be
externally individuated raise the related question about how best to
characterize the stimulations and behaviors that serve as inputs
and outputs to a system. Should they be construed as events
involving objects in a system's environment (such as fire trucks, water
and lemons), or rather as events in that system's sensory and motor
systems? Theories of the first type are often called
“long-arm” functional theories (Block, 1990), since they
characterize inputs and outputs — and consequently the states
they are produced by and produce — by reaching out into the
world. Adopting a “long-arm” theory would prevent our Twin
Earth counterparts from sharing our beliefs and desires, and may thus
honor intuitions that support an externalist individuation of
intentional states (though further questions may remain about what
Quine has called the “inscrutability of reference”; see
Putnam, 1988).

If functional characterizations of intentional states are intended
to capture their “narrow contents”, however, then the
inputs and outputs of the system will have to be specified in a way
that permits individuals in different environments to be in the same
intentional state. On this view inputs and outputs may be better
characterized as activity in specific sensory receptors and motor
neurons. But this (“short-arm”) option also restricts the
range of individuals that can share our beliefs and desires, since
creatures with different neural structures will be prevented from
sharing our mental states, even if they share all our behavioral and
inferential dispositions. (In addition, this option would not be open
to analytic functionalist theories, since generalizations that link
mental states to neurally specified inputs and outputs would not,
presumably, have the status of conceptual truths.)

Perhaps there is a way to specify sensory stimulations that
abstracts from the specifics of human neural structure enough to
include any possible creature that intuitively seems to share our
mental states, but is sufficiently concrete to rule out entities that
are clearly not cognitive systems (such as the economy of Bolivia; see
Block (1980)). If there is no such formulation, however, then
functionalists will either have to dispel intuitions to the effect that
certain systems can't have beliefs and desires, or concede that their
theories may be more “chauvinistic” than initially
hoped.

Clearly, the issues here mirror the issues regarding the
individuation of intentional states discussed in the previous section.
More work is needed to develop the “long-arm” and
“short-arm” alternatives, and to assess the merits and
deficiencies of both.

The previous sections were by and large devoted to the presentation
of the different varieties of functionalism and the evaluation of their
relative strengths and weaknesses. There have been many objections to
functionalism, however, that apply to all versions of the doctrine.
Some of these have already been presented in some detail, in
particular, the worry about the holism of standard functional
characterizations (discussed in section 4.2), and the worry (discussed
in sections 4.4. and 4.5) about whether any functional theory can
capture the representational content of intentional states. But there
are other important objections to functionalism in general that will be
addressed in detail here.

Even for those generally sympathetic to functionalism, there is one
category of mental states that seems particularly resistant to
functional characterization. Functionalist theories of all varieties
— whether analytic or empirical, FSIT or functional specification
— attempt to characterize mental states exclusively in
relational, specifically causal, terms. A common and persistent
objection, however, is that no such characterizations can capture the
qualitative character, or “qualia”, of
experiential states such as perceptions, emotions, and bodily
sensations, since they would leave out certain of their essential
properties, namely, “what it's like” (Nagel, 1975) to have
them. The next three sections will present the most serious worries
about the ability of functionalist theories to give an adequate
characterization of these states. (These worries, of course, will
extend to intentional states, if, as some philosophers have argued
(Searle, 1992, G. Strawson, 1986), “what it's like” to have
them is among their essential properties as well. See also entry on
Mental Representation.)

The first to be considered are the “absent” and
“inverted” qualia objections most closely associated with
Ned Block (1980b; see also Block and Fodor, 1972). The “inverted
qualia” objection to functionalism maintains that there could be
an individual who (for example) satisfies the functional definition of
our experience of red, but is experiencing green instead. It is a
descendant of the claim, discussed by philosophers from Locke to
Wittgenstein, that there could be an individual with an “inverted
spectrum” who is behaviorally indistinguishable from someone with
normal color vision; both objections trade on the contention that
purely relational characterizations cannot make distinctions among
distinct experiences with isomorphic causal patterns. (Even if inverted
qualia are not really a possibility for human beings, given certain
asymmetries in our “quality space” for color, and
differences in the relations of color experiences to other mental
states such as emotions (Hardin, 1988), it seems possible that there
are creatures with perfectly symmetrical color quality spaces for whom
a purely functional characterization of color experience would
fail.)

A related objection, the “absent qualia” objection,
maintains that there could be creatures functionally equivalent to
normal humans whose mental states have no qualitative character at all.
In his well-known “Chinese nation” thought-experiment,
Block (1980b) imagines that the population of China (chosen because its
size approximates the number of neurons in a typical human brain) is
recruited to duplicate his functional organization for a period of
time, receiving the equivalent of sensory input from an artificial body
and passing messages back and forth via satellite. Block argues that
such a “homunculi-headed” system — or
“Blockhead”, as it has come to be called — would not
have mental states with any qualitative character (other than the
qualia possessed by the individuals themselves), and thus that states
functionally equivalent to sensations or perceptions may lack their
characteristic “feels”. Conversely, it has also been argued
that functional role is not necessary for qualitative
character: for example, the argument goes, people may have mild, but
distinctive, twinges that have no typical causes or characteristic
effects.

All these objections maintain that it is possible for there to be
creatures with the functional organization of normal humans, but
without any, or the right sort, of qualia (or vice versa). In response,
some theorists (Dennett, 1978; Levin, 1985; Van Gulick, 1989) dispute
the claim that such creatures are possible. They argue that the
scenarios sketched to provide evidence of absent or inverted qualia are
clear-cut counterexamples only to crude examples of functional
definitions, and that attention to the subtleties of more sophisticated
characterizations will undermine the intuition that functional
duplicates of ourselves with absent or inverted qualia are possible
(or, conversely, that there are qualitative states without distinctive
functional roles). The plausibility of this line of defense is often
questioned, however, since there is tension between the goal of
increasing the sophistication (and thus the individuative powers) of
the functional definitions, and the goal of keeping these definitions
within the bounds of the a priori, which would be required for the
claim that it is inconceivable for there to be creatures with
absent or inverted qualia (but see the discussion in Section 4.3).

Another line of response, initially advanced by Sydney Shoemaker
(1994b), attributes a different sort of incoherence to the
“absent qualia” scenarios. Shoemaker argues that although
functional duplicates of ourselves with inverted qualia may be
possible, duplicates with absent qualia are not, since their
possibility leads to untenable skepticism about the qualitative
character of one's own mental states. This argument has been challenged
(Block, 1980b; see also Shoemaker's response in 1994d), but successful
or not, it raises questions about the nature of introspection and the
conditions under which, if functionalism is true, we can have knowledge
of our own mental states. These questions are discussed further in
section 6.

The “inverted” and “absent” qualia objections
were initially presented as challenges exclusively to functionalist
theories, both conceptual and empirical, and not generally to
physicalistic theories of experiential states; the main concern was
that the purely relational resources of functional description were
incapable of capturing the intrinsic qualitative character of states
such as feeling pain, or seeing red. (Indeed, in Block's 1980, p.291,
he suggests that qualitative states may best be construed as
“composite state[s] whose components are a quale and a
[functional state],” and adds, in an endnote (note 22) that the
quale “might be identified with a physico-chemical
state”.) But these objections are now generally regarded as
special cases of the “conceivability argument” against
physicalism, advanced by (among others) Kripke (1972) and Chalmers
(1996), which derives from Descartes's well-known argument in the
Sixth Meditation (1641) that since he can clearly and
distinctly conceive of himself existing apart from his body (and vice
versa), and since the ability to clearly and distinctly conceive of
things as existing apart guarantees that they are in fact distinct, he
is in fact distinct from his body.

Chalmers's version of the argument (1996), known as the
“Zombie Argument”, has been particularly influential. The
first premise of this argument is that it is conceivable, in a
special, robust, “positive” sense, that there are
molecule-for-molecule duplicates of oneself with no qualia (call them
“zombies”, following Chalmers, 1996). The second premise is
that scenarios that are “positively” conceivable in this
way represent real, metaphysical, possibilities. Thus, he concludes,
zombies are possible, and functionalism — or, more broadly,
physicalism — is false. The force of the Zombie Argument is due
in large part to the way Chalmers defends its two premises; he provides
a detailed account of just what is required for zombies to be
conceivable, and also an argument as to why the conceivability of
zombies entails their possibility (see also Chalmers and Jackson
(2002)). This account, based on a more comprehensive theory of how we
can have knowledge or justified belief about possibility and necessity,
reflects an increasingly popular way of thinking about these matters,
but remains controversial. (For alternative ways of explaining
conceivability, see Kripke (1986), Hart (1988); for criticism of
Chalmers's and Jackson's account, see Yablo (2001), Bealer (2002).)

In a related challenge, Joseph Levine (1983, 1993) argues that, even
if the conceivability of zombies doesn't entail that functionalism (or
more broadly, physicalism) is false, it opens an “explanatory
gap” not encountered in other cases of inter-theoretical
reduction, since the qualitative character of an experience cannot be
deduced from any physical or functional description of it. Such
attempts thus pose, at very least, a unique epistemological problem for
functionalist (or physicalist) reductions of qualitative states.

In response to these objections, analytic functionalists contend, as
they did with the inverted and absent qualia objections, that zombies
are not really conceivable, and thus there is no threat to
functionalism and no explanatory gap. But an increasingly popular
strategy for defending functionalism (and physicalism) against these
objections is to concede that there can be no conceptual analyses of
qualitative concepts (such as what it's like to see red or
what it's like to feel pain) in purely functional terms, and
focus instead on developing arguments to show that the conceivability
of zombies neither implies that such creatures are possible nor opens
up an explanatory gap.

One line of argument (Block and Stalnaker, 1999; Yablo, 2000)
contends that the conceivability of counterexamples to psycho-physical
or psycho-functional identity statements (such as zombies) has
analogues in other cases of successful inter-theoretical reduction, in
which the lack of conceptual analyses of the terms to be reduced makes
it conceivable, though not possible, that the identities are false.
But, the argument continues, if these cases routinely occur in what are
generally regarded as successful reductions in the sciences, then it's
reasonable to conclude that the conceivability of a situation does not
entail its possibility.

A different line of argument (Horgan, 1994; Loar, 1990; Lycan, 1990;
Hill, 1997) maintains that, while generally the conceivability of a
scenario entails its possibility, scenarios involving zombies stand as
important exceptions. The difference is that the qualitative, or
“what it's like”, concepts used to describe the properties
of experience that we conceive zombies to lack are significantly
different from the third-personal, discursive concepts of our common
sense and scientific theories such as mass, force
or charge; they comprise a special class of non-discursive,
first-personal, perspectival representations of those properties.
Whereas conceptually independent third-personal conceptsx and
y may reasonably be taken to express metaphysically
independent properties, or modes of presentation, no such metaphysical
conclusions can be drawn when one of the concepts in question is
third-personal and the other is qualitative, since these
concepts may merely be picking out the same properties in different
ways. Thus, the conceivability of zombies, dependent as it is on our
use of qualitative concepts, provides no evidence of their metaphysical
possibility.

Key to this line of defense is the claim that these special
qualitative concepts can denote functional (or physical) properties
without expressing some irreducibly qualitative modes of
presentation of them, for otherwise it couldn't be held that these
concepts do in fact apply to our functional (or physical) duplicates,
even though it's conceivable that they don't. This, not surprisingly,
has been disputed (White, 2002; Chalmers, 1999, but see the responses
of Loar, 1999, and Hill and McLaughlin, 1999; also see Levin, 2002, for
a hybrid view), and there is currently much discussion in the
literature about the plausibility of this claim. If this line of
defense is successful, however, it can also provide a response to the
“Distinct Property Argument”, discussed in section 2.5.

In another important, related, challenge to functionalism (and, more
generally, physicalism), Thomas Nagel (1974) and Frank Jackson (1982)
argue that a person could know all the physical and functional facts
about a certain type of experience and still not “know what it's
like” to have it. This is known as the “Knowledge
Argument”, and its conclusion is that there are certain
properties of experiences — the “what it's like” to
see red, feel pain, or sense the world through echolocation —
which cannot be identified with functional (or physical)
properties.

An early line of defense against these arguments, endorsed primarily
but not exclusively by conceptual functionalists, is known as the
“Ability Hypothesis”. (Nemirow, 1990, Lewis, 1990, Levin,
1986) “Ability” theorists suggest that knowing what it's
like to see red or feel pain is merely a sort of practical
knowledge, a “knowing how” (to imagine, remember, or
re-identify, a certain type of experience) rather than a knowledge of
propositions or facts. (See Tye, 2000, for a summary of the pros and
cons of this position.) An alternative, and currently more widespread,
view among contemporary functionalists is that coming to know what it's
like to see red or feel pain is indeed to acquire propositional
knowledge uniquely afforded by experience, expressed in terms of
first-personal concepts of those experiences. But, the argument
continues, this provides no problem for functionalism (or physicalism),
since these special first-personal concepts need not denote, or
introduce as “modes of presentation”, any irreducibly
qualitative properties. This view, of course, shares the strengths and
weaknesses of the analogous response to the conceivability arguments
discussed above.

There is one final strategy for defending a functionalist account of
qualitative states against all of these objections, namely,
eliminativism (Dennett, 1988; Rey, 1997). One can, that is, deny that
there are any such things as irreducible qualia, and maintain
that the conviction that such things do, or perhaps even
could, exist is due to illusion — or confusion.

Another important question concerns the beliefs that we have about
our own “occurrent” (as opposed to dispositional) mental
states such as thoughts, sensations, and perceptions. We seem to have
immediately available, non-inferential beliefs about these states, and
the question is how this is to be explained if mental states are
identical with functional properties.

The answer depends on what one takes these introspective beliefs to
involve. Broadly speaking, there are two dominant views of the matter
(but see Peacocke, 1999, Ch. 5 for further alternatives). One popular
account of introspection — the “inner sense” model on
which introspection is taken to be a kind of “internal
scanning” of the contents of one's mind (Armstrong, 1968) —
has been taken to be unfriendly to functionalism, on the grounds that
it's hard to see how the objects of such scanning could be second-order
relational properties of one's neural states (Goldman, 1993). Some
theorists, however, have maintained that functionalism can accommodate
the special features of introspective belief on the “inner
sense” model, since it would be only one of many domains in which
it's plausible to think that we have immediate, non-inferential
knowledge of causal or dispositional properties (Armstrong, 1993;
Kobes, 1993; Sterelney, 1993). A full discussion of these questions
goes beyond the scope of this entry, but the articles cited above are
just three among many helpful pieces in the Open Peer Commentary
following Goldman (1993), which provides a good introduction to the
debate about this issue.

Another account of introspection, identified most closely with
Shoemaker (1996a,b,c,d), is that the immediacy of introspective belief
follows from the fact that occurrent mental states and our
introspective beliefs about them are functionally interdefined. For
example, one satisfies the definition of being in pain only if one is
in a state that tends to cause (in creatures with the requisite
concepts who are considering the question) the belief that one is in
pain, and one believes that one is in pain only if one is in a state
that plays the belief role, and is caused directly by the pain itself.
On this account of introspection, the immediacy and non-inferential
nature of introspective belief is not merely compatible with
functionalism, but required by it.

But there is an objection, most recently expressed by George Bealer
(1997; see also Hill 1993), that, on this model an introspective belief
can only be defined in one of two unsatisfactory ways: either as a
belief produced by a (second-order) functional state specified (in
part) by its tendency to produce that very type of belief — which
would be circular — or as a belief about the first-order
realization of the functional state, rather than that state
itself. Functionalists have suggested, however (Shoemaker, 2001), that
there is a way of understanding the conditions under which beliefs can
be caused by, and thus be about, one's second-order functional states
that permits mental states and introspective beliefs about them to be
non-circularly defined. A full treatment of this objection involves the
more general question of whether second-order properties can have
causal efficacy, and is thus beyond the scope of this discussion (see
Mental Causation entry). But even if this objection can ultimately be
deflected, it suggests that special attention must be paid to the
functional characterizations of “self-directed” mental
states.

Yet another objection to functionalism raises the question of
whether any “theory” of the mind that invokes beliefs,
desires, and other intentional states could ever be, or even
approximate, an empirical theory. Whereas even analytic functionalists
hold that mental states are implicitly defined in terms of their
(causal or probabilistic) roles in producing behavior, these
critics take mental states, or at least intentional states, to be
implicitly defined in terms of their roles in rationalizing,
or making sense of, behavior. This is a different enterprise,
they claim, since rationalization, unlike causal explanation, requires
showing how an individual's beliefs, desires, and behavior conform, or
at least approximate, to certain a priori norms or
ideals of theoretical and practical reasoning —
prescriptions about how we should reason, or what, given our
beliefs and desires, we ought to do (Davidson, 1970, Dennett,
1978, McDowell, 1985). Thus the defining (“constitutive”)
normative or rational relations among intentional states expressed by
these principles cannot be expected to correspond to empirical
relations among our internal states, sensory stimulations and behavior,
since they comprise a kind of explanation that has sources of evidence
and standards for correctness that are different from those of
empirical theories (Davidson, 1970). One can't, that is, extract facts
from values.

Thus, although attributions of mental states can in some sense
explain behavior. by permitting an observer to “interpret”
it as making sense, they should not be expected to denote entities that
figure in empirical laws. (This is not to say, these theorists stress,
that there are no causes, or empirical laws of, behavior. These,
however, will be expressible only in the vocabularies of the
neurosciences, or other lower-level sciences, and not as relations
among beliefs, desires and behavior.)

Functionalists have replied to these worries in different ways. Many
just deny the intuition behind the objection, and maintain that even
the strictest conceptual analyses of our intentional terms and concepts
purport to define them in terms of their bona-fide causal roles, and
that any norms they reflect are explanatory rather than prescriptive.
They argue, that is, that if these generalizations are idealizations,
they are the sort of idealizations that occur in any scientific theory:
just as Boyle's Law depicts the relations between the temperature,
pressure, and volume of a gas under certain ideal experimental
conditions, our a priori theory of the mind consists of descriptions of
what normal humans would do under (physically specifiable)
ideal conditions, not prescriptions as to what they should, or
are rationally required, to do.

Other functionalists agree that we may advert to various norms of
inference and action in attributing beliefs and desires to others, but
deny that there is any in principle incompatibility between normative
and empirical explanations. They argue that if there are causal
relations among beliefs, desires, and behavior that even approximately
mirror the norms of rationality, then the attributions of intentional
states can be empirically confirmed (Fodor, 1990; Rey, 1997). In
addition, many who hold this view suggest that the principles of
rationality that intentional states must meet are quite minimal, and
comprise at most a set of constraints on the contours of our theory of
mind, such as that people can't, in general, hold (obviously)
contradictory beliefs, or act against their (sincerely avowed)
strongest desires (Loar, 1981). Still others suggest that the intuition
that we attribute beliefs and desires to others according to rational
norms is based on a fundamental mistake; these states are attributed
not on the basis of whether they rationalize the behavior in question,
but whether those subjects can be seen as using principles of inference
and action sufficiently like our own — be they rational, like
Modus Ponens, or irrational, like the Gambler's Fallacy. (See Stich,
1981, and Levin, 1988, for commentary)

But, though many functionalists argue that the considerations
discussed above show that there is no in principle bar to a
functionalist theory that has empirical force, these worries about the
normativity of intentional ascription continue to fuel skepticism about
functionalism (and, for that matter, any scientific theory of the mind
that uses intentional notions).

In the last part of the 20th century, functionalism stood as the
dominant theory of mental states. Like behaviorism, functionalism takes
mental states out of the realm of the “private” or
subjective, and gives them status as entities open to scientific
investigation. But, in contrast to behaviorism, functionalism's
characterization of mental states in terms of their roles in the
production of behavior grants them the causal efficacy that common
sense takes them to have. And in permitting mental states to be
multiply realized, functionalism offers an account of mental states
that is compatible with materialism, without limiting the class of
those with minds to creatures with brains like ours.

The sophistication of functionalist theories has increased since
their introduction, but so has the sophistication of the objections to
functionalism, especially to functionalist accounts of representation
(Sections 4.4, 4.5), the qualitative character of experiential states
(Section 5.1), and the nature of introspective knowledge (5.2). For
those unconvinced of the plausibility of dualism, however, and
unwilling to restrict mental states to creatures physically like
ourselves, the initial attractions of functionalism remain. The primary
challenge for future functionalists, therefore, will be to meet these
objections to the doctrine, either by articulating a functionalist
theory in increasingly convincing detail, or by showing how the
intuitions that fuel these objections can be explained away.